The invention relates to a method of treating siliceous formations to increase the permeability of the formations.
Subterranean sandstone or siliceous formations in oil and gas wells have been treated in the past with acid treatments to increase their permeability thus increasing or improving production from the formation. As used herein the term xe2x80x9csiliceousxe2x80x9d refers to the characteristic of having silica and/or silicate. Most sandstone formation are composed of over 70% sand quartz particles, i.e., silica (SiO2), bonded together by various amounts of cementing material including carbonate (calcite or CaCO3) and silicates.
The acid treatment of siliceous formations should be distinguished from the acid treatment of carbonate formations. Carbonate formations can be treated with a variety of acid systems, including hydrochloric, acetic and formic aids, with usually equal success. The treatment of siliceous formations with these acids, however, has little or no effect because they do not react appreciably with the silica and silicates which characterize the sandstone formations.
By far the most common method of treating sandstone formations involves introducing hydrofluoric acid into the wellbore and allowing the hydrofluoric acid to react with the surrounding formation. Hydrofluoric acid is preferred almost uniformly because of its reactivity with silica and silicates. The silicates include such things as clays and feldspars. Hydrofluoric acid tends to react very quickly with authigenic clays, such as smectite, kaolinite, illite and chlorite, especially at temperatures above 150xc2x0 F. as commonly found in subterranean formations. Because of this quick reaction, the hydrofluoric acid penetrates only a few inches of the formation before it is spent. Simultaneously, various alumina and/or silicate complexes precipitate as a result of the reaction of the hydrofluoric acid with the clays and silts. Such precipitation tends to plug pore spaces and reduce the porosity and permeability of the formation, thus impairing hydrocarbon flow potential. Further, because clays are normally a part of the cementitious material that holds the sandgrains of sandstone formations together, the dissolution of clay also weakens and unconsolidates the sandstone matrix in the vicinity of the wellbore, thus causing damage to the formation. The damaging effects due to both the unconsolidation of the matrix and the precipitation of complexes which clog the pore spaces of the formation can eliminate or even revert the stimulation effect of the acid treatment.
U.S. Pat. No. 5,529,125 teaches a useful method of treating siliceous or sandstone formations using hydrofluoric acid which reduces the amount of precipitates produced and which inhibits or retards the reactivity of the hydrofluoric acid with the clay or silicate elements of the formation. In this method, the formation is contacted with a treatment solution containing a phosphonate compound and a source of hydrofluoric acid.
However, certain siliceous formations contain aluminosilicates, such as zeolites, that are especially sensitive to hydrofluoric acid and tend to form undesirable reactions such as gelatinous precipitates in the presence of hydrofluoric acid. Such precipitates can clog pores of the formation thereby reducing the hydrocarbon productivity of the formation. Thus, a method of treating these especially sensitive formations is still needed.
A method of enhancing the productivity of hydrocarbons from hydrocarbon bearing siliceous formations, particularly those comprising aluminosilicates and most particularly those comprising zeolites, is achieved by contacting the formation with a treatment solution comprising citric acid, phosphonate compounds, and hydrofluoric acid. The advantages of this method are especially achieved when such treatment solution is applied prior to gravel packing or fracturing. This solution may optionally contain ammonium salts. Alternatively, similar solutions comprising citric acid and phosphonate compounds without hydrofluoric acid may be used to contact the formation prior to contacting the formation with the citric acid and phosphonate solution containing hydrofluoric acid. Other preflushes known in the art, such hydrochloric, acetic or formic acid solutions, may also be used.
Additional objects, features and advantages will be apparent in the written description which follows.